Conventional instruments for monitoring atmospheric phenomena local to the instruments use a number of different approaches. The monitored phenomena include all forms of liquid and frozen precipitation, such as rain, drizzle, snow and hail, and also include those suspended particles classed as obstruction to vision, namely mist, fog, haze, dust and smoke. These phenomena are hereinafter referred to as present weather.
A number of devices measure the rate of precipitation, also known as the amount or intensity of precipitation. One instrument utilizes two oscillating reservoirs or "tipping buckets" to collect precipitation in a liquid state. A heater melts frozen precipitation into water. This instrument has a lengthy reporting interval during light precipitation and may under-represent the amount of blowing precipitation. Other devices use optical or electrical techniques to measure velocity of precipitation. Particles falling through an electrical field alter capacitance while particles descending through two or more light beams trigger each beam in sequence.
Visibility meters are routinely used to quantify the obstruction to vision, or in other words to measure the visual range during periods of reduced visibility. Several basic types of visibility meters are commercially available for that purpose such as transmissometers, forward scatter meters, backscatter meters, and integrating nephelometers. None of these types of instruments measures visibility directly. In all cases the instruments measure the atmospheric extinction coefficient, or are calibrated to provide the extinction coefficient from a measurement of the scattering coefficient. Visual range is then calculated using Koschmieder's Law (or a variant thereof) for daylight situations and Allard's Law (or a variant thereof) for nighttime situations.
Visibility meters as stand-alone sensors cannot identify the atmospheric phenomena which produce reduced visibility. For example, if the relative humidity were high and the temperature were well below zero the atmospheric phenomenon producing reduced visibility could be identified as snow; one could not, however, distinguish between the various forms of snow, or between snow and freezing fog or rain during those rare conditions where it rains well below the freezing point. As another example, if the temperature were well above freezing accompanied by a high relative humidity, the possible causes of reduction in visibility could be reduced to rain or fog but one would not be capable of distinguishing between those two phenomena.
These capabilities offer an extremely limited present weather capability fraught with ambiguous identifications. Furthermore, they offer no possible means for measuring the rate or quantity of precipitation.
Several systems for present weather observation are fully automated and provide rapid response to present weather occurrences. One such system, the Laser Weather Identifier (LWI) has an optical transmitter and receiver. The light transmitter is a chopped CW He-Ne laser. In one version the receiver consists of three independently mounted detecting telescopes. One telescope looks directly at the projected laser beam. The other telescopes are off-axis and are aimed at the midpoint of the laser beam to detect light scattered through small angles (0.6.degree. and 1.2.degree.). This design permits detection and identification of precipitation using scintillation, extinction and off-axis forward scatter effects on the laser beam. Precipitation is detected, identified and quantified strictly by the amount of scintillation produced in the signal of the on-axis detector. The sole function of the two off-axis detectors is to detect and identify fog. The LWI has false alarm problems when strong winds are present because the turbulent air induces scintillations in the laser beam which are mistaken for precipitation. The LWI also has difficulty measuring snow.
Another system is known as the Precipitation Occurrence Sensor System (POSS). The POSS utilizes a commercially available Doppler Radar pointed vertically. The vertically pointed radar measures the descent velocity of precipitation within a few meters of the ground to detect the occurrence of precipitation.